purpose: Do a standard reduction of the input frame (debias, flatfielding), search for objects, determine their luminosities and align the derived photometry table with a standard star reference catalog.

warning:The twilight images of FORS2, which are used to create master sky flats, show evidence for a feature, whose position depends on the rotator angle (see overview). This limits the achievable photometric accuracy to about 5%. For more information please look at the FORS Secondary Standard and Absolute Photometry Project.

description: This recipe is used to estimate the magnitude zeropoint or the atmospheric extinction from one imaging exposure on a photometric standard stars field.

The bias master calibration is subtracted from the raw exposure. The debiased signal is then divided by the normalised sky flat field, and the overscan regions, if present, are removed from the result. The frame is not normalized 1 second exposure time. The calibrated image is then sent to a source detection and extraction application (SExtractor 2.5.0). The detected sources are compared to a catalogue of standard stars for identification. The comparison is made, whenever possible, applying point-pattern-matching techniques. If pattern matching either fails or is not applicable (e.g., too few standard stars are present in the field-of-view), then stars identification will be entirely based on the sky-to-CCD transformation specified in the input image FITS header. Finally, the difference between the catalog magnitude (corrected for the transmission curve difference between the used filter and the catalog filter, i.e. the colour term) and the instrumental magnitude (based on electron counts and corrected to airmass zero), is optimally averaged on all the identified standard stars. The derived quantity is conventionally referred to as the frame zeropoint. The corresponding extinction coefficient is computed as

E = Eph + (Zph - Z)/A

where Z is the computed frame zeropoint, Zph the expected zeropoint on a photometric night, Eph the atmospheric extinction coefficient of a photometric night (that was used in the computation of Z), and A is the airmass. The assumptions on extinction coefficients Eph and instrument zeropoints Zph for photometric nights are listed here for each FORS filter. They are also contained in the photometric tables ued by the pipeline and delivered to the PIs.

The zeropoint and the atmospheric extinction coefficients computed by this recipe have the sole purpose of monitoring the instrument+telescope system and the quality of the atmosphere. With only one exposure it is impossible to obtain actual determinations of either the atmospheric extinction coefficient (mag/airmass) or the instrument zeropoint without making assumptions on the other unknown. In order to evaluate them both we would need at least two different exposures of standard star fields (not necessarily of the same field), obtained at (very) different airmasses.